The invention relates to radio communications systems use I/Q-modulation, and more particularly to control of a power ratio between the I- and Q-channels in such a system.
Modulation schemes utilizing In-phase (I) and Quadrature (Q) signal components are known. In some cases, such as uplink transmissions in the IMT20000 Wideband Code Division Multiple Access (WCDMA) radio communications system standard proposed in Europe and Japan, IQ-modulation is used in a manner whereby different data channels are transmitted in the I and Q components (henceforth referred to throughout this disclosure as xe2x80x9cI-xe2x80x9d and xe2x80x9cQ-channelsxe2x80x9d). In the proposed WCDMA system, the common control channel (PCCH) is transmitted on the Q-channel at a data rate of 16 kilobits per second using a spreading factor of 256, while traffic and dedicated control channel (PDCH) is transmitted on the I-channel at a data rate somewhere between 32 kilobits per second (spreading factor of 128) and 1024 kilobits per second (spreading factor of 4).
The power requirements on each of these I- and Q-channels are of course different from one another. Therefore, before spreading and scrambling are applied, the I- and Q-channels have different power levels. It might first be assumed that the power of the channel will be proportional to the data rate in the channel. However, this is not necessarily the case because there are different quality of service requirements on the different channels. The PCCH channel has pilots that may require a quality of service that differs from that required for the speech or data services multiplexed on the PDCH-channel.
The power levels of the I- and Q-channels are controlled by a common Power Control algorithm. This algorithm increases or decreases the power in order to keep the signal power constant at the receiver. To accomplish this, the algorithm should follow the Rayleigh fading, the lognormal fading and the varying path loss due to the varying distance between the terminal and the base station.
A problem that is encountered derives from the requirement that the terminals in the WCDMA system have a transmitter characterized by good modulation accuracy. To get the exact power difference of, for example, 3 dB between the I and the Q-channels requires that the amplitude ratio in the terminal be   β  =            1              2              =          0.707      .      
To effect this power ratio, the data samples in the Q-channel are multiplied by the value of xcex2, and the resultant samples are supplied to spreading and complex modulation circuits, along with data samples from the I-channel.
Implementation of this exact power ratio is problematic because representation of the number 0.707 requires many bits that will need to participate in the multiplication that is performed on each sample that is to be transmitted. As is well-known, the computational burden imposed by a multiplication operation is related to the length of the operands involved. Increased computational burden translates not only into longer computational time, but also increased power requirements for performing the computation.
Moreover, when a system, such as the above-described WCDMA system, is designed with the assumption that the values of xcex2 may vary continuously, different components in the system (e.g., terminals made by different manufacturers) may introduce different quantization errors when representing xcex2. As a consequence of these mismatches, system performance will be degraded.
It is therefore an object of the present invention to provide a power ratio control strategy that improves performance compared to conventional techniques.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved in methods and apparatuses for use in a transmitter in a radiocommunications system such as a Wideband Code Division Multiple Access (WCDMA) communications system. The transmitter transmits one set of data on an in-phase (I-) channel and another set of data on a quadrature (Q-) channel. The transmitter generates a gain signal, xcex2, and multiplies the digital data associated with the Q-channel by the gain signal, xcex2. Complexity of the multiplication operation is reduced by limiting the gain signal, xcex2, to a finite number of values that are exactly representable by a predefined number of bits, such as 4-bit values to the right of a binary radix point.
In another aspect of the invention, modulation inaccuracy associated with quantization of xcex2 can be eliminated by utilizing the same quantized values of xcex2 in all components within the radiocommunications system.